n. WHAT IS A SPECIES ? 81 



■wliicli will prove acceptable to both classes of mind, — 

 acceptable alike to the technical observers and to the 

 logical reasoners. The definition given by M. De Can- 

 dolle is to some extent incomplete and even objectionable. 

 The first portion of it alludes only to resemblances be- 

 tween the individuals, which in their aggi'egate are deemed 

 to compose the species ; and it ignores their differences 

 from other species or groups of individuals, which always 

 enter largely into the practical conception of the species. 

 Secondly, as it is admitted that nearly allied species, and 

 also species of nearly allied genera (which ought to mean, 

 species less nearly allied) can mutually fertilize and pro- 

 duce fertile seeds, the test of fertility must be deemed so 

 far uncertain. It is thus imperfectly available, and even 

 occasionally deceptive, as a diagnostic test. The third is 

 a physiological character, equally or more variable and 

 exceptional than that of cross-breeding ; and it is one 

 which would occasionally separate varieties or garden 

 races more decidedly than species. The fourth is an 

 analogical summary of the others, rather than any addi- 

 tional character. 



A somewhat different definition will in consequence 

 here be substituted, or one somewhat differently ex- 

 pressed. Although it is allowed to be not much more 

 than a qualified expansion of the first portion of that 

 proposed by M. De Candolle, it appears to be more ap- 

 plicable to the actual practices of botanical systematists 

 and describers, and to be at the same time more closely 

 accordant with the abstract or ratiociuative idea of a spe- 

 cies. The definition will run thus : — 



A botanical species is a collective aggregate of indivi- 

 dual plants, closely resembling each other {firstly) in 

 having certain definable characters, common to all of 

 them, by which any of them may be recognised, and by 



